API Design

API Design

Purpose

Pick the right protocol for each API surface by traffic characteristics — not by fashion — and model resources, versioning, and async operations so the contract can evolve without breaking clients.

Universal — protocol selection criteria (latency, client shaping needs, internal vs external), resource modeling, and versioning strategy apply to any language; only the framework's routing/serialization differs.

Procedure

1. Choose protocol by traffic shape

   • REST — default for CRUD, public APIs, broad client compatibility, cacheable via HTTP
   • gRPC — internal service-to-service, sub-10ms latency mesh, streaming, strict contracts
   • GraphQL — clients need field-level shaping / aggregate many resources; accept the schema-design + N+1 discipline cost (requires a per-request batching/caching layer)
   • Decide by concrete thresholds (latency budget, payload variability), not "it depends"

2. Model resources (REST)

   • Nouns not verbs (/orders/{id} not /getOrder)
   • Nest only one level deep (/orders/{id}/items, not 4 levels)
   • Use sub-resources for relationships, query params for filtering

3. Design versioning before v1 ships

   • URL versioning (/v1/) — simplest, most visible
   • Header versioning — cleaner URLs, less discoverable
   • Commit to one; document the deprecation policy (sunset header + timeline)

4. Async / long-running operations

   • Return 202 Accepted + a status resource (/operations/{id}) for jobs > a few seconds
   • Don't hold the request open; pair with background-jobs skill

5. Validate the design

   • Walk the top 3 client use cases through the API — count round-trips
   • If a common screen needs 5+ calls → consider GraphQL or a BFF aggregate endpoint

Anti-patterns

Completion Criteria

• Protocol choice justified by a concrete traffic characteristic (documented)
• Versioning strategy decided and documented before v1
• Top 3 client use cases verified for round-trip count
• Long-running ops return 202 + status resource

Output

• API design doc / ADR: docs/adr/ADR-NNN-api-<surface>.md — protocol choice + rationale + versioning policy
• Route definitions: with resource modeling applied
• Commit format: feat(api): design <surface> endpoints [protocol: REST|gRPC|GraphQL]

Implementation

TypeScript + NestJS (default)

• REST: NestJS controllers + @nestjs/swagger for OpenAPI generation
• GraphQL: @nestjs/graphql (code-first) + DataLoader for N+1
• gRPC: @nestjs/microservices gRPC transport + .proto contracts
• Versioning: NestJS enableVersioning({ type: VersioningType.URI })

Other stacks

• Python / FastAPI: native OpenAPI; Strawberry/Ariadne for GraphQL; grpcio for gRPC
• Go: chi/echo for REST; gqlgen for GraphQL; google.golang.org/grpc
• Universal: resource modeling, versioning, and 202-async patterns are protocol-level, not framework-level

Related skills

• api-contract — once protocol is chosen, define the schema-first contract there
• schema-design — resource modeling aligns with DB schema
• authorization — per-resource access control

Reference

• Key insight encoded: Pick protocol by traffic shape — REST default for CRUD, gRPC for sub-10ms internal mesh, GraphQL only when clients need field-level shaping (and you accept the schema-design discipline).